1. Field of the Invention
The present invention relates to an ophthalmic photographing apparatus and more particularly to an exposure control system therefor. More specifically, the present invention pertains to an exposure control system for an ophthalmic photographing apparatus wherein the exposure is determined in accordance with the intensity of light reflected at the brightest part of an ophthalmic subject to be photographed, for example, a portion where focusing mark images are projected.
2. Description of Prior Art
In a conventional ophthalmic photographing instrument for example, an eye fundus camera, the exposure for photographing is generally determined by detecting the overall quantity of light reflected at the retina of a patient's eye by means, for example, a CdS detector. In other words, the exposure is determined in accordance with the average quantity of light reflected by the retina. It should nowever be noted that the intensity of the light reflected at the retina is not sufficient to precisely control the exposure since the reflectivity of the retina is very low.
On the other hand, the non-mydriatic eye fundus camera in which the retina is illuminated by an infrared beam for observation and focusing has recently come into wide use because it does not require projected administration of mydriatic to the pupil. With the non-mydriatic eye fundus camera, however, it is almost impossible to grasp the focus condition of the subject from the retina image on a monitoring TV since only a rough image can be produced on the TV. Therefore, an eye fundus camera is usually provided with a focusing mark projecting system for projecting images of a plurality of focusing marks on the retina through beams of infrared rays which are brighter than the background so that the operator can grasp the focus condition from the images of the focusing marks. It should therefore be noted that if exposure control is made based on the light reflected at the retina, the control is effected by the brighter light from the focusing mark images so that an accurate exposure control cannot be performed.